The field of the present invention is structural reinforcement in wood-frame buildings.
A need commonly exists to provide utilities, such as plumbing and electricity, to a wood-frame building. Often, main utility piping or wiring is routed into the attic or between floors of a building or other available space above an inhabited area. In order to route pipes and wiring from the attic or other upper space into a wall, a utility installer typically cuts or drills a hole in the portion of the wood-frame that forms the top of the wall--the pieces of lumber referred to as the top plate.
While this hole in the top plate is necessary to provide utilities to the structure, the hole also can significantly weaken or fully disrupt the top plate. As the hole increases in diameter, the strength of the top plate decreases and the shear strength of the entire structure may be adversely affected. Where cut completely, the top plate loses the ability to transfer force in tension or in sheer across the cut.
One of the ways in which low-level wood-frame structures are damaged and destroyed in earthquakes, tornadoes and hurricanes is through the failure of walls placed in shear under the load. Structural engineers often strengthen portions of the walls of such a structure to provide greater resistance to shear only at spaced locations. The shear forces on the wall are then transferred along the top plate to these strengthened portions. Typically, top plates run continuously across a wood-frame building in both directions so as to provide appropriate strength against shear. Failure of the top plate at any one point can defeat the transfer of shear forces such that the wall can fail in shear. A weakening of the top plate by either cutting or drilling through it for the passage of utilities, therefore, weakens the entire wall.
Heretofore, metal straps have been used which are placed to one or both sides of the top plate adjacent the hole or cut in recognition of the loss of structural integrity at such locations. Such metal straps typically act quite well in tension but provide little support in either compression or in shear. Another solution has been to bolt an angle iron to the inside of the top plate in these areas. The substantial extension of one of the flanges outwardly from the structure has a significant detrimental impact on the placement of wall board, other surface features and juxtaposed structural elements. The angle iron does contribute to strength in compression as well as tension.